Are all fossil animals dinosaurs?

No. Dinosaurs are a group of ancient reptiles that had a set of particular skeletal features. The hips,
hind legs, and ankles were specialized and allowed the legs to move directly under the body, rather than
extending out from the side of the body as in modern lizards. This arrangement enabled dinosaurs to bring
their knees and ankles directly below their hips and provided the necessary attachments for very strong
leg muscles. Dinosaur skeletons were well designed for supporting a large body, for standing erect
(upright), and for running. The front legs were adapted for grasping prey, for supporting weight, or
for walking and running. The skulls of dinosaurs were designed for maximum strength, for minimum weight,
and (in some cases) for grasping, holding, or tearing at prey. These skeletal features separated dinosaurs
from other ancient reptiles such as Dimetrodon, the plesiosaurs, and pterosaurs. Fossil mammals,
like mammoths and "saber-toothed tigers" (e.g., Smilodon), are also often incorrectly called
dinosaurs.

These ancient animals are NOT dinosaurs!

Did people and dinosaurs live at the same time?

No! After the dinosaurs died out, nearly 65 million years passed before people
appeared on Earth. However, small mammals (including shrew-sized primates) were
alive at the time of the dinosaurs. Many scientists who study dinosaurs (vertebrate
paleontologists) now think that birds are direct descendants of one line of
carnivorous dinosaurs, and some consider that they in fact represent modern
living dinosaurs. This theory remains under discussion and shows that there is
still much we don't know about dinosaurs.

Where did dinosaurs live?

Paleontologists now have evidence that dinosaurs lived on all of the
continents. At the beginning of the age of dinosaurs (during the Triassic Period,
about 230 million years ago) the continents we now know were arranged together
as a single supercontinent called Pangea. During the 165 million years of
dinosaur existence this supercontinent slowly broke apart. Its pieces then spread
across the globe into a nearly modern arrangement by a process called plate tectonics.
Volcanoes, earthquakes, mountain building, and sea-floor spreading are all part
of plate tectonics, and this process is still changing our modern Earth.

Relative positions of continents during the age of dinosaurs

Did all the dinosaurs live together, and at the same time?

Dinosaur communities were separated by both time and geography. The "age of
dinosaurs" (the Mesozoic Era) included three consecutive geologic time
periods (the Triassic, Jurassic, and Cretaceous Periods). Different dinosaur
species lived during each of these three periods. For example, the Jurassic
dinosaur Stegosaurus already had been extinct for approximately 80 million
years before the appearance of the Cretaceous dinosaur Tyrannosaurus. In
fact, the time separating Stegosaurus and Tyrannosaurus is
greater than the time separating Tyrannosaurus and you. At the beginning
of dinosaur history (the Triassic Period), there was one supercontinent on Earth
(Pangea). Many dinosaur types were widespread across it. However, as Pangea broke
apart, dinosaurs became scattered across the globe on separate continents, and new
types of dinosaurs evolved separately in each geographic area.

How are dinosaurs named?

Dinosaurs generally are named after a characteristic body feature, after the
place where they were found, or after a person involved in the discovery. Usually
the name consists of two Greek or Latin words (or combinations); in order, these
are the genus (plural, genera) and the species name. For example, the Greek and
Latin combination (binomen) Tyrannosaurus rex means "king of the tyrant
lizards." Biologists name modern animals exactly the same way. Some examples
include humans (Homo sapiens), domestic dogs (Canis familiaris),
golden eagles (Aquila chrysaetos), box turtles (Terrapene carolina),
and rattlesnakes (Crotalus horridus).

What was the biggest dinosaur? What was the smallest?

The largest complete dinosaur we know of was Brachiosaurus ("arm lizard");
it reached 23 m in length and 12 m in height (about the length of two large school
buses and the height of a four-story building). Fragmentary leg bones and
vertebrae of even larger dinosaur species are known, but these skeletal remains
are too incomplete to determine their exact size. Several of these
(Argentinasaurus and Amphicoelias) might have been one and a half
to two times larger than Brachiosaurus. The smallest dinosaurs were just
slightly larger than a chicken; Compsognathus ("pretty jaw") was 1 m (3
ft) long and probably weighed about 2.5 kg (about 6.5 lb). These three dinosaur
types all lived during the Jurassic Period. Mussaurus ("mouse lizard") was
claimed as the smallest dinosaur, but it is now known to be the hatchling of a
dinosaur type that was much larger than Compsognathus when fully grown. If
birds are advanced dinosaurs, then the smallest dinosaur would be the
hummingbird!

How many types of dinosaurs are known?

Approximately 700 species have been named. However, a recent scientific review
suggests that only about half of these are based on fairly complete specimens that
can be shown to be unique and separate species. These species are placed in about
300 valid dinosaur genera (Stegosaurus, Diplodocus, etc.), although
about 540 have been named. Recent estimates suggest that about 700 to 900
more dinosaur genera may remain to be discovered.

Most dinosaur genera presently contain only one species (for example,
Deinonychus) but some have more (for example, Iguanodon). Even if
all of the roughly 700 published species are valid, their number is still less
than one-tenth the number of currently known living bird species, less than
one-fifth the number of currently known mammal species, and less than one-third the
number of currently known spider species.

Were dinosaurs warm-blooded?

Scientists have conflicting opinions on this subject. Some paleontologists
think that all dinosaurs were "warm-blooded" in the same sense that modern birds
and mammals are: that is, they had rapid metabolic rates. Other scientists think
it unlikely that any dinosaur could have had a rapid metabolic rate. Some
scientists think that very big dinosaurs could have had warm bodies because of
their large body size, just as some sea turtles do today. It may be that some
dinosaurs were warm-blooded. The problem is that it is hard to find evidence that
unquestionably shows what dinosaur metabolisms were like.

How long could a dinosaur live?

Animal lifespans relate in part to their body size and in part to their type of
metabolism. Dinosaur lifespans probably varied in length from tens of years to
hundreds of years. Their possible maximum age can be estimated from the maximum
lifespans of modern reptiles, such as the 66-year lifespan of the common
alligator (Alligator mississippiensis) and the impressive lifespan of a
Black Seychelles Tortoise (Geochelone (Aldabrachelys) sumeirei). One
specimen of this now-extinct species, which was an adult when captured, lived a
record 152 years in captivity (1766-1918) and had an accidental death. These
estimates, based on lifespans of cold-blooded animals, would be too long if
dinosaurs had metabolisms more similar to modern birds and mammals.

What did dinosaurs eat?

Some dinosaurs ate lizards, turtles, eggs, or early mammals. Some hunted
other dinosaurs or scavenged dead animals. Most, however, ate plants (but
not grass, which hadn't evolved yet). Rocks that contains dinosaur bones
also contain fossil pollen and spores that indicate hundreds to thousands
of types of plants existed during the Mesozoic Era. Many of these plants had
edible leaves, including evergreen conifers (pine trees, redwoods, and their
relatives), ferns, mosses, horsetail rushes, cycads, ginkos, and in the latter
part of the dinosaur age flowering (fruiting) plants. Although the exact time of
origin for flowering plants is still uncertain, the last of the dinosaurs
certainly had fruit available to eat.

How fast could dinosaurs walk or run?

Estimates of dinosaur speeds vary because several different methods are used to
calculate them. One recent estimate suggests that an average person might have been
able to to outrun an adult Tyrannosaurus (although you probably would not
volunteer to try). The two basic approaches for estimating dinosaur speed are
comparing to recorded speeds of modern animals of similar body size and build,
and measuring distances between fossil footprints in a trackway and using these
distances to calculate estimated speed. Walking-speed estimates for medium-sized
bipedal (two-legged) dinosaurs vary from 4 kph to 6 kph, and peak running-speed
estimates vary from 37 kph to 88 kph. The highest figure (88.6 kph) is the same
as the peak speed of the currently fastest land animals, such as the North
American pronghorn "antelope" (Antilocapra americana), and very probably
is too high.

Speed comparison of some ancient and modern animals

Did dinosaurs communicate?

Dinosaurs probably communicated both vocally and visually. The chambered
headcrests on some dinosaurs such as Corythosaurus and
Parasaurolophus might haved been used to amplify grunts or bellows.
Defensive posturing, courtship behavior, and territory fights probably involved
both vocal and visual displays. An angry Triceratops bull shaking his head
at you, even silently, would have made himself very clearly understood!

Why did some dinosaurs grow so big?

Paleontologists don't know for certain, but perhaps a large body size protected
them from most predators, helped to regulate internal body temperature, or let
them reach new sources of food (some probably browsed treetops, as giraffes
do today). No modern animals except whales are even close in size to the largest
dinosaurs; therefore, paleontologists think that the dinosaurs' world was much
different from the world today and that climate and food supplies must have
been favorable for reaching great size.

Which was the smartest dinosaur?

Although there is no direct way to measure a dinosaur's intelligence, one of the
few possible measures of intelligence might be a large brain in a small body.
The genus that perhaps fits this description best was the Cretaceous bird-like
dinosaur Troodon, which also may have had binocular vision (depth
perception) and excellent eyesight and was built for speed. Even so, this
dinosaur was probably not as "intelligent" as most modern birds and mammals.

Ceratosaurus nasicornis, from Marsh (1896)
Orginal specimen on display at U.S. National Museum

What colors were dinosaurs?

Direct fossil evidence for dinosaur skin color is unknown. Paleontologists think
that some dinosaurs likely had protective coloration, such as pale undersides
to reduce shadows, irregular color patterns ("camouflage") to make them less
visible in vegetation, and so on. Those dinosaurs that had enough armor, such as
the stegosaurs and ceratopsians, may not have needed protective coloration but
may have been brightly colored as a warning to predators or as a display for
finding a mate. Most dinosaurs probably were as brightly colored as modern
lizards, snakes, or birds.

Were dinosaurs social animals?

Some dinosaurs were social creatures. Recently discovered evidence indicates
that they travelled together and that some may even have migrated (because
dinosaur fossils have been found above the Arctic Circle, where food supply would
have been seasonal). Grouped hadrosaur nest sites have been found with badly
crushed eggshells and skeletons of baby dinosaurs (with slightly worn teeth)
still in the nests, suggesting that some babies stayed in their nests after
hatching and probably were fed by parents.

When did dinosaurs become extinct?

Dinosaurs went extinct about 65 million years ago (at the end of the Cretaceous
Period), after living on Earth for about 165 million years. If all of Earth
time from the very beginning of the dinosaurs to today were compressed into
365 days (1 calendar year), the dinosaurs appeared January 1 and became extinct
the third week of September. (Using this same time scale, the Earth would have
formed approximately 18.5 years earlier.) By comparison, people (Homo
sapiens) have been on earth only since December 31 (New Year's eve). The
dinosaurs' long period of dominance certainly makes them unqualified successes
in the history of life on Earth.

Why did the dinosaurs die out?

There are dozens of theories to explain a probable cause or causes. Throughout
the Mesozoic Era, individual dinosaur species were evolving and becoming extinct
for various reasons. The unusually massive extinction at the end of the
Cretaceous exterminated the last of the dinosaurs, the flying reptiles, and the
large swimming reptiles, as well as many other marine animals. There is now
widespread evidence that a meteorite impact was at least the partial cause for
this extinction. Impact craters are visible on most planets in our solar system.
A spectacular example of this was witnessed in 1994, when Jupiter was struck by
a series of cometary fragments. Some of these impact blasts were larger than the
Earth's diameter. Other factors such as extensive release of volcanic gases,
climatic cooling (with related changes in ocean currents and weather patterns),
sea-level change, low reproduction rates, poison gases from a comet, or changes
in the Earth's orbit or magnetic field may have contributed to this extinction
event.

Where can I find more information about dinosaurs?

Your local museums, public libraries, and bookstores are good places to start.
Some national monuments (Dinosaur National Monument, UT and CO), national parks
(Petrified Forest National Park, AZ), and state parks (for example, Dinosaur
Valley State Park, TX) have outstanding displays. State geological surveys also
have or can provide information on nearby dinosaur exhibits. The references below
will help you get started; they provided some of the information for this
pamphlet.

Dodson, P., and Dawson, S.D., 1991, Making the fossil record of dinosaurs:
Modern Geology, vol. 16, p. 3-15.